Met-Proxies - Metal isotopes as environmental proxies

Our understanding on the evolution of Earths climate in the past relies on the chemical and isotopic signals recorded in geological archives. Indeed environmental conditions such as temperature, water pH, atmospheric CO2 content and biological activity, amongst others, are recorded in the chemical and isotopic composition of calcium carbonate minerals forming in water bodies like lakes or the ocean. In order to read these chemical signals however, the appropriate tools are required that correlate the composition of the geological archives with the prevailing environmental conditions. In Met-Proxies project we are actually developing such tools. In order to provide precise information on the pH and the biological activity during the time of carbonate mineral formation in the geological past, state-of-the-art laboratory mineral formation and novel isotope techniques will be combined. More specifically in this project we examine the incorporation as well as the isotopic fractionation of copper and zinc during carbonate mineral formation. Moreover in order to validate the findings of the experimental work, a field study that includes chemical and isotopic analyses in the speleothems of Katerloch cave will be conducted. The expected outcome of the Met-Proxies project is to provide the scientific community with new and more precise tools that estimate pH, atmospheric CO2 partial pressure and biological activity in natural waters. Moreover during this project a doctoral and two master students will have the opportunity to get training in geochemical techniques and field work.

During carbonate mineral formation in natural systems such lakes, caves or marine environments, a number of trace metals can be entrapped in the crystal structure. Proper quantification of these foreign elements in these carbonate minerals can represent valuable indicators of the environmental conditions prevailing at the time of mineral formation such as temperature, pH or organic carbon content. The results of this project have shown experimentally that the incorporation of foreign elements into carbonate minerals is dependent on (1) the structure of the considered mineral (organization of the elements within the mineral), (2) the physical characteristics of the foreign element (e.g. the size of the element), and (3) the formation conditions (e.g. mineral growth rate, temperature). The work carried out in this project also supported the hypothesis that foreign elements that have highly incompatible characteristics with the structure of the mineral in which they are incorporated are probably located in positions where the structure of the mineral is imperfect (known as defect sites). In addition, the acquisition of new experimental data allowed comparisons with theoretical calculations of elemental incorporation existing in the literature. It appears that our results show very different values for elements that are highly incompatible with the mineral structure, underlining the necessity of developing new theoretical incorporation models for these elements. Indeed, understanding the parameters affecting the presence of an element into a given carbonate mineral will enable us to understand the characteristics of the environment in which this mineral was formed and therefore the environmental conditions that prevailed in the past. This work has also highlighted the fundamental differences that can exist between the incorporation of trace elements into carbonate minerals when they are precipitated by organisms or not. For example, one of the most robust markers of seawater temperature is the Li/Mg ratio in carbonate minerals produced by corals and/or foraminifera. Interestingly, the results of this project shown that the Li/Mg ratio is only slightly affected by temperature compared with other parameters such as mineral growth rate. These findings raise questions about the processes by which organisms select particular elements during the formation of biogenic carbonate. Finally, the incorporation of transition metals (usually dangerous for living organisms when present in excessively high concentrations in the environment) into carbonate minerals, and the increase in storage with the mineral growth rate, opens up new areas of application in the remediation of polluted water/soil.